This invention relates to the production of high purity hybrid cabbage seeds, and more particularly concerns a method of producing high purity F.sub.1 hybrid cabbage seeds in high yield without the prior necessity of labor-intensive bud pollination to produce the parent plants or lines.
The current procedures used to produce F.sub.1 hybrid cabbage seeds are widely recognized as having serious limitations, both in terms of cost and as regards seed purity. They all require the establishment and provision of stable, sib-incompatible, nearly homozygous, parental breeding lines, which are available after repeatedly selfing each genotype. Because cabbage are strongly self-incompatible (an individual plant will not readily pollinate with itself), inbreeding to maintain the parental lines can only be effected by bud pollination; each flower bud is opened by hand, and then pollinated from another flower on the same plant, again by hand.
These difficulties in maintaining the parental inbred breeding lines are reflected not only in high hybrid seed cost as a result of the labor cost, but, in practical effect, have required that the breeding lines be maintained in foreign low-labor-cost countries.
Moreover, the parental lines are of necessity highly inbred, and the plants have low vigor, resulting in low F.sub.1 hybrid seed yields.
When making hybrid seed from two parental lines, either of two approaches are used. For one, the flowers in one line are emasculated by hand, and are fertilized with pollen from the crossing line. This manifestly is labor intensive, and consequently expensive. A second approach, widely used on a commercial scale, avoids emasculation. Two inbred parental lines, where one or both lines is sib-incompatible (does not readily pollinate with its siblings) as well as self-incompatible (nearly incapable of fertilizing itself) are crossed by natural pollination. Hybrid seed is collected only from the sib-incompatible breeding line. In the usual case, where one inbred is sib-incompatible, a high ratio, typically 3:1, of the sib-incompatible inbred to the normal inbred is used, and only the seed produced on the sib-incompatible inbred is harvested for sale. Ideally, if both inbreds are sib-incompatible, a 1:1 ratio of the two can be used, and, due to the reciprocity rule of genetics, the entire seed crop can be harvested.
While the sib-incompatible technique for F.sub.1 hybrid cabbage seed production avoids many of the difficulties of emasculation, a number of problems are encountered in practice. One such problem is that large populations of inbreds are used, and these tend to drift genetically due to their being sexually reproduced.
An especially serious seed quality problem arises with the use of sib-incompatible hybrid techniques. If a "misnick" (poor timing between the two inbreds in their going into inflorescence) occurs, some sibbing takes place on the ostensibly sib-incompatible parent, giving rise to inbreds being present in the seed crop. This may result in the seed not being of sufficiently high purity to comply with present day labeling laws, which mandate that a hybrid seed contain at least 95% of the designated hybrid whereas self-incompatibility, being the stronger reaction, greatly reduces this problem.
And finally, the development of a sib-incompatible inbred normally requires at least about 10 years of inbreeding.